1. Field of the Invention
The present invention relates to a predistortion linearizer and predistortion distortion compensation method, a program, and a medium which are used at communication base stations for mobile units such as cellular phones.
2. Related Art of the Invention
In recent years, transmitters at base stations for mobile communication equipment have required very efficient and linear power amplifiers in order to collectively amplify a large number of signal channels. To improve the linearity of a power amplifier, a predistortion linearizer based on, for example, a predistortion system must be employed.
Here, the configuration and operation of a conventional predistortion linearizer will be described below with reference to FIG. 10, which is a block diagram thereof.
In FIG. 10, reference numeral 601 denotes an input terminal, 602 is an output terminal, 603 is a power divider, 604 is a delay circuit, 605 is a distortion generating circuit, 606 is a variable attenuator, 607 is a variable phase shifter, 608 is a power synthesizer, 609 is a power amplifier, 610 is a directional coupler, and 611 is a control section.
Such a circuit configuration is disclosed in, for example, Japanese Patent Laid-Open No. 2000-261252. In this case, intermodulation distortion that may occur in the power amplifier 609 can be reduced by controlling the variable attenuator 606 and the variable phase shifter 607 so that the power amplifier 609 receives, as an input, a signal having the same amplitude (dBc value) as an intermodulation distortion (IM) component that may occur in the power amplifier 609 as well as an opposite phase relative to this component.
Then, an intermodulation distortion characteristic observed if two sine waves of frequencies f1 and f2 (f1<f2) are input to the distortion generating circuit 605 and the power amplifier 609, respectively, will be described with reference to FIGS. 11(a) to 11(d).
FIG. 11(a) is a chart illustrating the distortion amplitude characteristic of the distortion generating circuit 605. FIG. 11(b) is a chart illustrating the distortion phase characteristic of the distortion generating circuit 605. FIG. 11(c) is a chart illustrating the distortion amplitude characteristic of the power amplifier 609. FIG. 11(d) is a chart illustrating the distortion phase characteristic of the power amplifier 609. (The axes of abscissas in FIGS. 11(a) and 11(b) indicate the output power of the distortion generating circuit 605 (the amplifier thereof), whereas the axes of abscissas in FIGS. 11(c) and 11(d) indicate the output power of the power amplifier 609. Further, a dBm unit is used on the axes of ordinates in FIGS. 11(a) and 11(c), whereas a deg unit is used on the axes of ordinates in FIGS. 11(b) and 11(d)).
When the frequency of an intermodulation distortion component generated on the low frequency side of the frequency f1 is defined as f3 and the frequency of an intermodulation distortion component generated on the high frequency side of the frequency f2 is defined as f4, these frequencies have different distortion amplitude characteristics and different distortion phase characteristics. That is, the distortion amplitude and phase characteristics of the distortion generating circuit 605 and power amplifier 609 depend on an output level.
It is common that the distortion generating circuit and the power amplifier have different distortion characteristics.
However, in the configuration of the conventional predistortion linearizer (see FIG. 10), the operation level of the distortion generating circuit 605 varies in proportion to the operation level of the power amplifier 609. Thus, it has been difficult to compensate for distortion over a wide range of output levels.
More specifically, as shown in FIGS. 11(a) to 11(d), if for example, the operation level of the power amplifier 609 changes from P1 to P2 (for example, substantially half of P1), the operation level of the distortion generating circuit 605 changes from P3 to P4 (for example, substantially half of P3).
Distortion is effectively compensated for because the distortion level observed if the operation level of the power amplifier 609 is P1 substantially equals the distortion level observed if the operation level of the distortion generating circuit 605 is P3 (that is, the difference in graph value for distortion amplitude or phase between the frequencies f3 and f4 at the former operation level equals the difference in graph value for distortion amplitude or phase between the frequencies f3 and f4 at the latter operation level). However, since the distortion level observed if the operation level of the power amplifier is P2 significantly differs from the distortion level observed if the operation level of the distortion generating circuit is P4 (that is, the difference in graph value at the former operation level differs from the difference in graph value at the latter operation level), distortion is not sufficiently compensated for regardless of the manner in which vector adjustment is carried out in the variable attenuator 606 and variable phase shifter 607.